Polychlorinated biphenyls (PCBs) are a family of compounds that contain a biphenyl nucleus with multiple chlorine substitutions. Approximately 209 different chlorinated biphenyls are possible. This is the result of the ten positions available for chlorine substitution on the biphenyl ring structure. In PCB mixtures, each of the ten may be occupied by either chlorine or hydrogen. PCBs were manufactured by Monsanto between 1929 and 1978 under the trade name Aroclor. Various complex mixtures of PCBs were manufactured and typically differed only in the percent of chlorine by weight. In most cases, the PCB compounds were identified with a numerical designation beginning with the number 12. The number 12 signified the 12 carbons of the biphenyl ring. The designation was typically completed with two additional numbers indicating the percentage by weight of chlorine present. For example, Aroclor 1254 represents a biphenyl ring structure wherein number 54 represents 54% chlorine by weight. Aroclor 1242 represents a biphenyl ring structure wherein number 42 represents 42% chlorine by weight.
During the time in which they were manufactured, PCBs were industrially important organic chemicals with diverse applications. These compounds were used as coolants and dielectric fluids in transformers and in capacitors, heat transfer fluids, coatings for wood products, flame retardants and chlorinated pesticides.
Unfortunately, PCBs are extremely stable and difficult to degrade. The widespread distribution during the forty-five years of their manufacture coupled with their stability has lead to an accumulation of PCBs in the environment.
Because of the environmental problems associated with PCBs, manufacturing, distribution and use of PCBs have essentially been banned in the United States since 1976. Although it has been years since the production of PCBs ceased in the United States, these chemicals are still widely distributed in the environment, especially in the soil and sediments.
In several regions, the presence of PCBs in the soil and sediments present a particular problem because PCBs tend to bioaccumulate. For example, in the Great Lakes region, PCBs have been found in fish and fish products. In addition, recently the distribution of PCBs in breast milk of Canadian women has been reported.
Due to their stability and associated toxicity, PCBs are environmental pollutants which are of major concern. The clean-up of Sites contaminated with PCBs presents a major challenge. Several biological methods have been explored as potential means for reducing the presence of PCBs in the environment.
Generally aerobic bacteria degrade only biphenyl and lower chlorinated PCBs. That is, aerobic bacteria degrade biphenyl and biphenyl rings having one to four chlorine substitutes i.e., mono- to tetra-chlorine substitutes. A few aerobic microbial species can dechlorinate and mineralize defined congeners of PCBs containing up to four chlorines. See, Harkness, et al. "In Situ Stimulation of Aerobic PCB Biodegradation In Hudson River Sediments", Science, Vol. 259, Jan. 22, 1993, pp. 503-507 and Abramowicz, "Aerobic and Anaerobic Biodegradation of PCBs: A Review", Critical Reviews In Biotechnology, Vol. 10, Issue 3, pp. 241-251 (1990). Aerobic organisms that have been used in an attempt to dechlorinate PCBs include common soil bacteria and complex fungi.
Dechlorination of more highly chlorinated PCBs, that is PCBs with up to 10 chlorine substitutions have been observed under anaerobic conditions. Hence, the anaerobic processes look very attractive for bioremediation of PCB-contaminated soils and sediments and in particular for the dechlorination of highly chlorinated PCBs. See, for example, Quensen, et al., "Dechlorination of Four Commercial Polychlorinated Biphenyl Mixtures (Aroclors) by Anaerobic Microorganisms From Sediments", Applied and Environmental Microbiology, August, 1990 p. 2360-2369.
Quensen, et al. used sediments containing anaerobic microorganisms obtained from various sites to dechlorinated PCBs in solution. However, the microorganisms could not be cultivated without sediments as pure or mixed cultures and the addition of large amounts of sediment to lakes and rivers containing PCB contaminated sediment is not a practical solution.
A two-step process involving both anaerobic and aerobic treatment has also been suggested in order to dechlorinate PCBs. This process involves the use of anaerobic and aerobic steps in sequence to dechlorinate highly chlorinated PCBs to biphenyls.
In the past, complete dechlorination of PCBs has been difficult to achieve. In fact, although selective dechlorination of the meta and para chlorines of PCBs has been accomplished by various mechanisms and reported in numerous instances, ortho dechlorination of PCBs has been generally unsuccessful. However, ortho dechlorination of a PCB in solution using slurries of sediment containing bacteria has been reported by Van Dort and Bedard, "Reductive ortho and meta Dechlorination of a Polychlorinated Biphenyl Congener by Anaerobic Microorganisms", Applied and Environmental Microbiology, May, 1991, p. 1576-1578. Obviously, the use of slurries of sediment containing bacteria to treat lake and river sediment containing PCBs has disadvantages over the use of concentrated consortia of such bacteria.
Presently, there is no completely effective method for treatment of PCB-contaminated sediments and soils. There is certainly no effective in situ method of treating contaminated sediments which results in a complete dechlorination of the highly chlorinated biphenyl ring.
Granules containing microorganisms have been used to treat waste. The advantages of using such granules are that they do not contain any amount of sediments, they are easy to handle, they are stable and they usually stay where they are placed.
In 1991, the use of granules for treatment of waste that contained pentachlorophenol (PCP), trichloroethylene (TCE) and perchloroethylene (PCE) was described in Bhatnagar et al., "Design and Function of Biomethanation Granules for Hazardous Waste Treatment", In Proceedings, International Symposium on Environmental Biotechnology, p. 1-10, Royal Flemish Society of Engineers, Belgium, (1991). The design and use of a dechlorinating biomethanation (DSB) consortia for treatment of pentachlorophenol (PCP) was described. For treatment of PCP, the biomethanation (SB) granules-were modified by adding mixed acidogenic and methanogenic cultures enriched on PCP. These modified cultures were found to satisfactorily degrade pentachlorophenol (PCP) See Wu et al., "Performance of Anaerobic Granules for Degradation of Pentachlorophenol", Applied Environmental Microbiology, vol. 59, 1993, pp. 389-397. However, subsequent use of those same granules in an attempt to degrade polychlorinated biphenyls (PCBs) proved disappointing. Although some dechlorination occurred, it was far from complete.
It would be advantageous to have granules that contain a consortium of microorganisms that can dechlorinate PCBs and methods of using such granules to dechlorinate PCBs in the sediment of rivers and lakes in situ.